It has heretofore been disclosed in Patent Document 1 a composition for a sliding member in which polyamide-imide resin, polyamide resin, polyimide resin, epoxy resin or phenol resin is used as the binder resin, and further fluorinated polymer such as polytetrafluoroethylene (PTFE), etc.; molybdenum disulfide or tungsten disulfide is used as the solid lubricant. A technique which uses graphite as the solid lubricant has been disclosed in Patent Document 2.
However, to give lubricity to the coating film by using the solid lubricant is inconsistent with adhesiveness of the coating film to the ground, and there is a problem that lubricity is to be improved, then, adhesiveness is necessarily lowered.
When a formulation ratio of the solid lubricant is a little, lubricity is not sufficient and it is not suitable as a composition for a sliding member. To the contrary, when a formulation ratio of the solid lubricant is large, adhesiveness is lowered, and in particular, at the high load region, the coating film is easily peeled off from the ground.
Depending on the uses, it has been attempted to establish the function by optimizing the amounts of these solid lubricants to take a balance between adhesiveness and lubricity, or to improve adhesiveness by modifying the ground using a primer treatment. However, in the former, there is a limit to be consistent, and in the latter, remarkable improvement in adhesiveness cannot be expected.
To the contrary, in Patent Document 3, it has been attempted to establish the function by markedly improve lubricity by using silicone oil, but there is a problem that adhesiveness of the coating film is lowered by using the liquid lubricant.
In Patent Document 4, PAI (polyamide-imide) is baked at a lower layer, and PAI using a solid lubricant is coated thereon, but the following problems are involved.
(1) A drying process is necessary. A process must be added, so that it takes a cost as compared with the wet-on-wet coating.
(2) The layers are completely separated between the lower layer and the upper layer, so that adhesiveness between the layers is lowered, or the solid lubricant is not contained in the lower layer portion, so that the upper layer is worn away then sliding property is lost.(3) There is no benefit by mixing due to compatibility of the upper layer and the lower layer.(4) When PAI is used in the lower layer alone in the wet-on-wet coating, difference in shrinkage between the lower layer and the upper layer is large, so that crack is easily generated at the coated surface.